In 1849, Fizeau calculated a value for the speed of light to a better precision than the previous value determined by Ole Rømer in 1676. He used a beam of light reflected from a mirror 8 kilometers away. The beam passed through the gaps between teeth of a rapidly rotating wheel. The speed of the wheel was increased until the returning light passed through the next gap and could be seen.

Fizeau calculated the speed of light to be 313,300 kilometres per second (194,700 mi/s), which was within 5% of the correct value (299,792.458 kilometers per second). Fizeau published the first results obtained by his method for determining the speed of light in 1849. (See Fizeau–Foucault apparatus.)[5] Fizeau made the first suggestion in 1864 that the "speed of a light wave be used as a length standard".[6]

Fizeau was involved in the discovery of the Doppler effect,[7] which is known in French as the Doppler–Fizeau effect.